On the season two premiere of Science Solved It, we dive into one of the most pervasive conspiracy theories: chemtrails.

Chemtrails—the theory that the white streaks seen behind airplanes are part of a secret government spraying program—is one of the most pervasive conspiracy theories out there.

A recent study found 10 percent of Americans “completely” believe in chemtrails, while up to 30 percent think there’s at least some truth to the theory. But there’s a relatively simple explanation for where these streaks of white come from, so why does this theory persist?

Let’s start by debunking the name itself. What conspiracy theorists call chemtrails are actually called contrails, short for condensation trails. It’s simply water vapor condensing in the air to make man-made clouds.

“Jet fuel is a form of hydrocarbons,” explained Ken Caldeira, a climate scientist at the Carnegie Institution for Science, in the latest episode of our podcast Science Solved It. “And when you burn the hydrogen, it oxidizes to form water, and when you burn carbon it forms carbon dioxide. Mostly what you’re seeing is water vapor, much of which came out of the back of the engine and then condensed into kind of an artificial cloud.”

Planes traveling at lower altitudes, below 26,000 feet, typically can’t produce contrails because the surrounding air isn’t cold enough. This is why we sometimes see trails behind planes, sometimes don’t, and sometimes notice that they vanish soon after appearing. Conspiracy theorists claim this is evidence of a spraying program, but it’s really just evidence that the air varies based on altitude and geography.

“If I was flying in Alaska, sometime during the winter you can actually make contails with a small plane because it's really to do with the temperature of the air around you and not to do with the plane itself,” said Mick West, a retired video game programmer and pilot who runs a website called MetaBunk, that debunks popular conspiracy theories. “If you happen to have a car that was driving on the top of Mount Everest, you could make a contrail with that car.”

There’s also no evidence whatsoever that anything else is going on. In fact, in 2016, Caldeira published a survey of the world’s leading atmospheric scientists, including atmospheric chemists and geochemists. The survey found that 76 of the 77 participating scientists found no evidence of a spraying program and that all the alleged evidence could easily be explained by other factors. The one outstanding scientist attributed the inexplicable evidence to “high levels of atmospheric barium in a remote area with standard 'low' soil barium.” So, not exactly a smoking gun.

Why, then, does the theory persist? Part of the problem is that the US government and scientists have a long history of lying to the public about dangerous chemicals and performing secret experimentation, so a healthy skepticism of what Uncle Sam tells you is harmless is not necessarily a bad thing.

The theory also gained more steam in recent years as the public learned more about proposals for geoengineering: the concept of intentional, global manipulation of environmental processes to try to curb the effects of global warming. Though it’s not currently in practice, scientists like Caldeira have used models to show that geoengineering could be used as a potential tool to reduce the global temperature. And that possibility has convinced many to believe that it’s already in effect. The problem is, if you think the government is manipulating the climate, you’re probably not as likely to get on board with actual solutions for climate change. And that kind of thinking can impact your daily life.

“I got an email from a woman who wouldn't let her kids play outside, because she was fearful that if they played outside the chemicals would drift down from these chemtrails and poison her children,” Caldeira told Motherboard. “It's really kind of tragic to think that there's people sitting indoors in fear, afraid to play out in their backyard. I want to [spread awareness] in part to help her children have a better life, because nobody should be told to stay inside afraid of going outside.”